Polyester resin (particularly, polyethylene terephthalate, polyethylene 2,6-naphthalene dicarboxylate, etc.) is superior in mechanical properties, thermal characteristics, chemical resistance, electrical properties and moldability, and is used for various uses. Polyester film produced by forming the above-described polyester into film, especially biaxially oriented polyester film, is used for solar cell back sheet, electrical insulation material for water heater motor, car air conditioner motor used for hybrid cars, electric insulation material for driving motors, magnetic recording material, material for capacitors, material for packing, material for construction, uses for photography, uses for graphics, and uses for thermal transfer printing.
Above all, especially for electrical insulation material (for example, used for car air-conditioner motor and driving motor) for which high heat-resistance and high moist-heat resistance are required, because films consisting only of polyester do not have sufficient heat resistance and moisture-heat resistance, JP-A-62-292431 coats both surfaces of polyester film with polyphenylene sulfide layers. Generally, polyester hydrolysis under moist-heat atmosphere proceeds by using COOH end group as a self-catalyst, and the quantity of COOH end group further increases as a result of the polyester hydrolysis. Because the hydrolysis further proceeds as the quantity of COOH end group increases, it is necessary to curb the increase of COOH end group under moist-heat atmosphere. JP-A-2010-248492 and WO 2010/103945 A curb the increase of COOH end group of polyester under moist-heat atmosphere by adding phosphoric acid and alkali metal phosphate salt to improve moist-heat resistance of a film comprising polyester compositions. Similarly, WO 2011/052290 A curbs the increase of COOH end group of polyester under moist-heat atmosphere by adding phosphoric acid and alkali metal phosphate salt to improve moist-heat resistance of a film comprising polyester compositions. JPA-2003-306536, JP-A-2008-81533, JP-A-2006-265792 and JP-A-2000-309649 improve the resistance of polyester by controlling the ratio between the amount of phosphorus compounds and the amount of metals. JP-A-2011-119651 and WO 2005/026241 A add pigments to polyester.
However, those curbs on the increase of COOH end group under moist-heat atmosphere are not sufficient, and in a long-term use, there is a concern that it is not possible to keep mechanical properties and electrical insulation property because polyester hydrolysis proceeds. In JP '492, WO '945, WO '290, JP '536, JP '533 and JP '792, the metal elements to be contained in polyester are inadequate to improve moist-heat resistance, and the moist-heat resistance of the obtained polyester composition is not sufficient. In JP '649 and JP '651, the phosphorus compounds to be contained in polyester are inadequate to improve moist-heat resistance, and the moist-heat resistance of the obtained polyester composition is not sufficient. In WO '241, neither metal elements or phosphorus compounds are added for improving moist-heat resistance of polyester, and the moist-heat resistance of the polyester composition is not sufficient.
Accordingly, it could be helpful to provide a polyester composition having an excellent moist-heat resistance by curbing the increase of COOH end group under moist-heat atmosphere, a process of producing same, a film using same, a sheet-like structure comprising a laminated structure using same film, an electrical insulation sheet having an excellent heat resistance and having an excellent moist-heat resistance by using same sheet-like structure, and processes for producing same.